Almost all process difficulties occasioned by laminate inconsistencies can be explained by some failure of control in the manufacture of the laminates. Laminates, as the name implies, are composed of layers of materials united by a binding material, Thermosetting laminates are pressed together under heat and pressure to form a dense, consistent, electrical insulator. The basic raw materials of thermosetting laminates are paper, glass, cloth, or glass mat, bonded together with various thermosetting resins, such as phenolic, epoxy or polyester resins.
The first operation in laminate manufacture usually consists of impregnating or coating a base material, such as paper, glass cloth, or other fibrous sheet materials with a thermosetting resin. The resin is partially cured by heating to a point of cure suitable for storage. The base material is passed through a dip tank of resin, then into a set of squeeze rollers and through a drying oven. The resin is partially cured to what is called a B stage, and most of the volatiles in the resin are driven off in the oven. The partially polymerized or semi-cured material is also known as "prepreg", and in general is dry and tack-free.
Rigid process control is maintained during treating in order to monitor the ratio of resin to base material, the final thickness of the prepreg, and the degree of resin cured. However, laminate pressing is both manually controlled and operated. The most modern procedure requires the control of a skilled operator employing a skill based on years of experience to determine operating variables, the most critical of which is the determination of when to apply full laminating pressure to the heated laminate assembly. Usually, uncontrollable variables and differences in judgment have been such that no two operators would follow the same pattern, and seldom would the same operator use an identical pattern on successive batches. The high cost involved, together with the high incidence of poor interlaminar bonds, and the necessity for uniformity, as well as the unavailability of skilled operating personnel have long indicated the desirability of scientific control and automation.
In the process of laminating thermosetting laminates, the point at which full laminating pressure is applied to the heated laminate assembly is critical. This criticality is occasioned by the nature of the thermosetting resins which are irreversible when polymerized. Partially polymerized or semi-cured thermosetting resins liquefy when first heated. The resin then passes to a gel stage and finally sets or polymerizes as a hardened material. The thermosetting resin cannot be liquefied again by heating. Therefore, it is important in setting up a pressing cycle of a laminating process to apply full laminating pressure while the resin is in the gel stage. The premature application of full laminating pressure while the resin is in the liquid stage will result in excess resin being squeezed out of the laminate assembly leaving voids in the finished laminate. Poor interlaminar bonds also result from the application of full laminating pressure when the resin has passed from the gel to the solid stage since there is insufficient resin flow.
Attempts to insure successful interlaminar bonds have primarily involved modification in the press employed. The modification usually resides in the use of a press which is capable of rapid temperature rise. In essence, the pressing cycle is started at room temperature, and the temperature is raised to the gel point of the resin, at which point full laminating pressure is applied. The gel point is ascertained by imbedding a thermocouple in the center sheet of pregreg. This modification while overcoming some of the inherent disadvantages of the process by providing longer gel stages, requires operation by a skilled technician. Furthermore, the incidence of poor interlaminar bonds is not significantly reduced since the temperature at which a particular resin achieves its gel stage is not constant and is reliant on the temperature history of the B stage of the resin.
Accordingly, it is a primary object of the present invention to provide an improved laminating process which enables the accurate determination of the gel stage of a thermosetting resin so as to insure the application of full laminating pressure at the proper time in the laminate pressing step.
Another object of the present invention is to provide an improved process for laminating multilayer circuit laminates.
It is a further object of the present invention to provide an improved press which is adapted to electrically determine the gel stage of a thermosetting resin.
Yet another object of this invention is to overcome such inherent disadvantages of the prior art laminating process as poor interlaminar bonds, interior voids in the laminate, and the like.
Additionally, it is a further object of the invention to provide successful interlaminar bonds in a multilayer circuit laminate without resorting to roughing the printed, metallic circuit pattern surfaces of the fully cured laminate circuit layers by etching or mechanical abrasion prior to lamination.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.